In Fig. REF , we compare our design with instantaneous CSI-based design, which
estimates the instantaneous channels in each coherence interval as [1]}. With single user, the minimum pilot length is \(1+N\)  in instantaneous CSI scheme[1]}. Therefore, its rate (red line) should be multiplied by a loss factor \( 1-\frac{1+N}{\tau _{c}}\) . Meanwhile, we consider an ideal case (green line) with a loss factor \(1-\frac{1}{\tau _{c}} \) , which is the same loss factor of our scheme. We can see that if we ignore the additional rate loss in channel estimation (green line), using instantaneous CSI can achieve better performance, especially in large \(N\)  regime. However, when considering the overhead in pilot transmission (red line), the instantaneous CSI scheme first increase but then decrease, which performs worse than our design. This is because instantaneous CSI scheme need longer time for channel estimation, which is proportional to \(N\) . Hence, when \(N\)  is large, only little time is left for data transmission.
However, our design can keep improving the rate performance as increasing \(N\) . Therefore, compared with instantaneous CSI-based design, statistical CSI-based design can realize better performance but with lower overhead and complexity in practical systems.
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